1. Field of the Invention
The present invention relates to a method for tinting a water-absorptive contact lens. More particularly, it relates to a method whereby it is possible to certainly apply tinting to the predetermined surface of the lens.
2. Discussion of Background
The tinting of contact lenses brings about some merits such that it is effective for protecting the users from bright light, it makes it easy to find contact lenses when lost, and it serves to improve the image of the commercial products. Thus, it has been in practice to slightly tint entire lenses or to tint only desired portions of the lenses. Further, marks such as letters, designs or symbols may be tinted at desired portions of contact lenses to identify them, whereby classification or grouping of lenses may be facilitated by the identification of contact lens standards, or contact lenses for right and left eyes can be distinguished, so that the users may be prevented from erroneously applying the contact lenses to opposite eyes.
The tinting of marks such as letters or designs is effective especially for water-absorptive contact lenses, since it is thereby possible to readily distinguish the front and rear sides. Namely, such water-absorptive contact lenses become soft upon absorption of water, whereby the convex and concave sides of such lenses are likely to be readily inverted in such a swelled state (in a water-containing state or a hydrated state) and it is very difficult to distinguish the front and rear sides since the convex side can not necessarily be regarded as the front side of the lens, although such soft contact lenses have excellent characteristics such that they provide a comfortable wearing feeling to the users and they are highly safe to the eyes. By tinting a mark such as letters, it is possible to distinguish the front and rear sides of a contact lens quite easily, since the side on which the mark is tinted can be identified as the front side of the lens.
As a tinting or marking method for such a water-absorptive contact lens, there have been proposed a method wherein marking is conducted by irradiating a laser beam to a contact lens in a swelled state (Japanese Examined Patent Publication No. 37368/1987, a method wherein a dye solution is applied to a contact lens in a swelled state, followed by irradiating ultraviolet rays thereto to fix the dye solution (Japanese Examined Patent Publication No. 10045/1989) and a method wherein a contact lens in a swelled state is tinted by means of a tinting agent containing a vat dye and a reducing agent (Japanese Unexamined Patent Publication No. 26471/88. In each case, the marking is applied to a contact lens in a swelled state (in a water-containing state). As described above, with a water-absorptive contact lens, the convex and concave sides are likely to be readily inverted in the swelled state, and if the tinting operation is conducted in the inverted state, the dye solution which should be applied to the front side of the lens may erroneously be applied to the rear side of the lens.
Once a mark is applied to a lens, the lens will be handled as indicated by the mark. Therefore, with a mis-marked contact lens, the user is likely to use the contact lens erroneously with the front and rear sides inverted in accordance with the mis-marking, whereby there will be a problem that the user feels a pain or an uncomfortable wearing feeling, not to mention that a proper lens function can not be thereby obtained.